The present invention relates generally to a single pass full color printing system and, more particularly, to a color printing system consisting generally of a raster output scanner (ROS) optical system and a quad-level xerographic unit and a tri-level xerographic unit in tandem which can print pixels producing black and white and all six primary colors.
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The electrostatic charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a bi-level latent charge pattern on the imaging surface where the high charge regions correspond to the areas not exposed by radiation. One level of this charge pattern is made visible by developing it with toner. The toner is generally a colored powder that adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface, or is transferred to a receiving substrate such as plain paper, to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, upon exposure, three charge levels are produced on the charge-retentive surface. The highly charged (i.e. unexposed) areas are developed with toner, and the area more fully discharged is also developed, but with a toner of a different color. Thus, the charge retentive surface contains three exposure levels; zero exposure, intermediate exposure, and full exposure, which correspond to three charge levels. These three levels can be developed to print, for example, black, white, and a single color.
FIG. 1 is a schematic drawing of a prior art tri-level xerographic printing system. As shown, the system utilizes a charge retentive member in the form of a photoconductive belt 10, consisting of a photoconductive surface on an electrically conductive, light-transmissive substrate mounted for movement past a charge station A, an exposure station B, developer station C, transfer station D, and cleaning station F. Belt 10 moves in the direction of arrow 16 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about a plurality of rollers 18, 20 and 22, the former of which can be used as a drive roller, and the latter of which can be used to provide suitable tensioning of the photoreceptor belt 10. Motor 23 rotates roller 18 to advance belt 10 in the direction of arrow 16. Roller 18 is coupled by motor 23 by suitable means such as a belt drive.
As can be seen by further reference to FIG. 2, initially successive portions of belt 10 pass through charging station A, where a corona discharge device such as a scorotron, corotron, or dicorotron, indicated generally by the reference numeral 24, charges the belt 10 to a selectively high uniform positive or negative potential, V.sub.0. Any suitable control circuit, as well known in the art, may be employed for controlling the corona discharge device 24.
Next, the charged portions of the photoreceptor surface are advanced through exposure station B. At exposure station B, the uniformly charged surface of belt 10 is exposed by a tri-level raster output scanner (ROS) unit 25, which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. This scan results in three separate discharge regions on the photoreceptor, each region exposed at one of three possible levels: (1) zero exposure which results in a voltage equal to V.sub.ddp and will be developed using charged-area-development (CAD); (2) full exposure, which results in a low voltage level V.sub.C and is developed using discharged-area-development (DAD); and (3) intermediate exposure, which yields an intermediate voltage level V.sub.W and does not develop and yields a white region on the print. These voltage levels are shown schematically in FIG. 2. Some typical voltage levels are as follows.
The photoreceptor, which is initially charged to a voltage V.sub.0, undergoes dark decay to a level V.sub.ddp (V.sub.CAD) equal to about -900 volts. When exposed at the exposure station B, the photoreceptor is discharged to V.sub.c, (V.sub.DAD) equal to about -100 volts in the highlight (i.e. color other than black) color portions of the image. The photoreceptor is also discharged to V.sub.w (V.sub.white) equal to -500 volts imagewise in the background (i.e. white), image areas and in the inter-document area. Thus the image exposure is at three levels; zero exposure (i.e. black), intermediate exposure (white) and full exposure (i.e. color). After passing through the exposure station, the photoreceptor contains highly charged areas and fully discharged areas which correspond to CAD and DAD color latent images, and also contains an intermediate level charged area that is not developed.
At development station C, a development system, indicated generally by the reference numeral 30, advances developer materials into contact with the CAD and DAD electrostatic latent images. The development system 30 comprises first and second developer housings 32 and 34. The developer housing 32 contains a pair of magnetic brush rollers 35 and 36. The rollers advance developer material 40 into contact with the photoreceptor for developing the charged-area regions (V.sub.CAD). The developer material 40, by way of example, contains positively charged black toner. Electrical biasing is accomplished via power supply 41, electrically connected to developer apparatus 32. A suitable DC bias, V.sub.bb, of approximately -600 volts is applied to the rollers 35 and 36 via the power supply 41.
The developer housing 34 contains a pair of magnetic rolls 37 and 38. The rollers advance developer material 42 into contact with the photoreceptor for developing the discharged-area regions (V.sub.DAD). The developer material 42, by way of example, contains negatively charged red toner. Appropriate electrical biasing is accomplished via power supply 43 electrically connected to developer apparatus 34. A suitable DC bias, V.sub.cb, of approximately -400 volts is applied to the rollers 37 and 38 via the bias power supply 43.
Because the composite image developed on the photoreceptor consists of both positive and negative toner, a positive pre-transfer corona discharge member (not shown) is provided to condition the toner for effective transfer to a substrate, using positive corona discharge. The pre-transfer corona discharge member is preferably an AC corona device, biased with a DC voltage to operate in a field sensitive mode, to perform tri-level xerography pre-transfer charging in a way that selectively adds more charge (or at least comparable charge) to the region of the composite tri-level image that must have its polarity reversed. This charge discrimination is enhanced by discharging the photoreceptor carrying the composite developed latent image with light before the pre-transfer charging this minimizes the tendency to overcharge portions of the image which are already at the correct polarity.
Referring again to FIG. 1, a sheet of support material 58 is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by conventional sheet feeding apparatus, not shown. Preferably, the sheet feeding apparatus includes a feed roll contacting the upper most sheet of a stack of copy sheets. Feed rolls rotate to advance the uppermost sheet from the stack into a chute, which directs the advancing sheet of support material into contact with the surface of belt 10 in a timed sequence, so that the developed toner powder image contacts the advancing sheet of support material at transfer station D.
Transfer station D includes a corona generating device 60 which sprays ions of a suitable polarity onto the backside of sheet 58. This attracts the charged toner powder images from the belt 10 to sheet 58. After transfer, the sheet continues to move in the direction of arrow 62 onto a conveyor (not shown) which advances the sheet to fusing station E.
Fusing station E includes a fuser assembly, indicated generally by the reference numeral 64, which permanently affixes the transferred powder image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 and backup roller 68, with the toner powder image contacting fuser roller 66. In this manner, the toner powder image is permanently affixed to sheet 58. After fusing, a chute, not shown, guides the advancing sheet 58 to a catch tray (also not shown), for subsequent removal from the printing machine by the operator.
After the sheet of support material is separated from the photoconductive surface of belt 10, the residual toner particles carried by the nonimage areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station F. A magnetic brush cleaner housing is disposed at the cleaner station F. The cleaner apparatus comprises a conventional magnetic brush roll structure for causing carrier particles in the cleaner housing to form a brush-like orientation relative to the roll structure and the charge retentive surface. It also includes a pair of detoning rolls for removing the residual toner from the brush.
Subsequent to cleaning, a discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining, prior to the charging thereof, for the successive imaging cycle. An illustrative example of a tri-level xerographic unit is found in U.S. Pat. No. 4,990,955, assigned to the same assignee as the present invention and herein incorporated by reference.
There are several scanning techniques known in the prior art to obtain the tri-level exposure imaging. A conventional flying spot scanner, such as used in the Canon 9030 uses a ROS unit to "write" an exposed image on a photoreceptive surface a pixel at a time. To obtain higher spatial resolution, a pulse imaging scanner can be utilized. This pulse imaging scanner is also referred to as a Scophony scanner in an article in Optical Engineering, Vol. 24, No. 1, January/February 1985, Scophony Spatial Light Modulator, by Richard Johnson et al., whose contents are hereby incorporated by reference. A preferred technique, capable of higher spatial resolution is to use similar optical elements as the flying spot scanner (rotating polygon, laser light source, pre-polygon and post-polygon optics), but with an A/O modulator which illuminates many pixels at a given time, resulting in a scanner with a coherent imaging response. With this type of scan system, the exposure level, or levels at the image surface, can be controlled by controlling the drive level of the A/O modulator dependent on the video data. In a tri-level system, two drive levels are used, one for the white exposure and a second higher drive level for the DAD exposure.
Alternately, instead of obtaining an intermediate exposure level by controlling the acoustic amplitude, an intermediate exposure can be provided by using pulse width modulation in a pulse imaging system in conjunction with spatial filtering.
In quad-level or four-level color imaging, upon exposure, four charge levels are produced on the charge-retentive surface. Thus, the charge retentive surface contains four exposure levels; zero exposure, a low intermediate exposure, a high intermediate exposure and full exposure, which correspond to the four charge levels. These three levels can be developed to print, for example, black, white, and two colors.
FIG. 3 is a schematic drawing of a prior art quad-level xerographic printing system 100. As shown, the system utilizes a charge retentive member in the form of a photoconductive belt 110, consisting of a photoconductive surface on an electrically conductive, light-transmissive substrate mounted for movement past a charge station A, an exposure station B, developer station C, transfer station D, and cleaning station F. Belt 110 moves in the direction of arrow 112 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 110 is entrained about a plurality of rollers 114, and 116, the former of which can be used as a drive roller, and the latter of which can be used to provide suitable tensioning of the photoreceptor belt 110. Motor 118 rotates roller 114 to advance belt 110 in the direction of arrow 112. Roller 114 is coupled by motor 118 by suitable means such as a belt drive.
As can be seen by further reference to FIG. 4, initially successive portions of belt 110 pass through charging station A, where a corona discharge device such as a scorotron, corotron, or dicorotron, indicated generally by the reference numeral 120, charges the belt 110 to a selectively high uniform positive or negative potential, V.sub.0. Any suitable control circuit, as well known in the art, may be employed for controlling the corona discharge device 120.
Next, the charged portions of the photoreceptor surface are advanced through exposure station B. At exposure station B, the uniformly charged surface of belt 110 is exposed by a quad-level raster output scanner (ROS) unit 122, which causes the charge retentive surface to be discharged in accordance with the output from the scanning device.
The photoreceptor belt 110, which is initially charged to a voltage V.sub.black (approximately minus 1000 volts), is discharged to V.sub.w (approximately minus 700 volts) imagewise in the background (white) image areas and to V.sub.d (approximately minus 350 volts) and V.sub.a (approximately minus 100 volts) in the highlight (i.e. colors other than black) image areas.
At development station C in FIG. 3, a development system, indicated generally by the reference numeral 124, advances developer materials into contact with the electrostatic latent images on the photoreceptor belt 110. The development system 124 comprises first, second and third developer housings 126, 128 and 130. Preferably, each of the developer housings contains a pair of magnetic brush rollers 132 and 134. These rollers advance their respective developer materials into contact with the latent image.
By way of example, the first developer housing 126 contains positively charged black toner, the second developer housing 128 contains negatively charged magenta toner and the third developer housing 130 contains negatively charged cyan toner. The photoreceptor belt 110 containing the images from the exposure station B and the ROS unit 122 is moved past these housings 126, 128 and 130 in a single pass.
Color discrimination in the development of the electrostatic latent image is achieved by electrically biasing the three housings to suitable voltages for effecting the attraction of the desired toners to the different potentials on the charge retentive surface of the photoreceptor belt. Thus, the first developer housing 126 may be biased by a DC voltage, V.sub.b, to approximately minus 800 volts, the second developer housing 128 may be biased by a DC voltage, V.sub.c1, to approximately minus 300 volts and the third developer housing 130 may be biased by a DC voltage, V.sub.c2, to approximately minus 600 volts.
The black toner from the first developer housing 126 is attracted to the V.sub.black voltage areas on the photoreceptor and repelled from the other two charged areas, V.sub.d and V.sub.a. The positively charged black toner from the first developer housing 126 is attracted to the V.sub.black voltage areas on the photoreceptor belt which are at a charge level of minus 1000 volts since the bias on the first developer housing 126 is minus 800 volts. The positively charged black toner is attracted to the photoreceptor areas which are more negative than the developer housing. Conversely the positively charged black toner from the first developer housing is not attracted to the photoreceptor areas, V.sub.d (approximately minus 350 volts) and V.sub.a (approximately minus 100 volts), that are more positive than the first developer housing bias of minus 800 volts.
The magenta toner from the second developer housing 128 is attracted to the V.sub.a voltage areas on the photoreceptor and repelled from the other two charged areas, V.sub.d and V.sub.black. The voltage level V.sub.a of minus 100 volts is less negative than the minus 300 volts of the second developer housing and the negative charge of the magenta toner. The magenta toner is not attracted to the photoreceptors areas of voltage levels V.sub.d of minus 350 volts because these areas are more negative than the minus 300 volts bias of the second developer housing and thus repell the magenta toner.
The cyan toner from the third developer housing 130 is attracted to both the V.sub.a and the V.sub.d voltage areas on the photoreceptor. The voltage levels of V.sub.d of minus 350 volts and V.sub.a of minus 100 volts are both more positive than the minus 600 volts bias of the third developer housing 130 and the negatively charged cyan toner.
Thus, the V.sub.black voltage areas on the photoreceptor attracts the black toner from the first developer housing 126 to produce a black color image. The V.sub.d voltage areas on the photoreceptor attracts the cyan toner from the third developer housing 130 to produce a cyan color image. The V.sub.a voltage areas photoreceptor attracts the magenta toner from the second developer housing 128 and the cyan toner from the third developer housing 130 to produce a blue color image. The areas of the photoreceptor charged to V.sub.w of minus 700 volts are not developed by any of the toners because the biasing of the toner housings and the polarities of the toners.
Thus, the quad-level xerographic unit 100, where the voltages of the color highlight areas on the photoreceptor and the color developer housing biases are between the white voltage level and ground, will produce black, white, cyan (the color of the toner whose housing bias is closest to white) and blue (a mixture of cyan and magenta).
Because the composite image developed on the photoreceptor consists of both positive and negative toner, a positive pre-transfer corona discharge member 136 is provided to condition the toner for effective transfer to a substrate, using positive corona discharge. The pre-transfer corona discharge member is preferably an AC corona device, biased with a DC voltage to operate in a field sensitive mode, to perform xerography pre-transfer charging in a way that selectively adds more charge (or at least comparable charge) to the region of the composite image that must have its polarity reversed. This charge discrimination is enhanced by discharging the photoreceptor carrying the composite developed latent image with light before the pre-transfer charging this minimizes the tendency to overcharge portions of the image which are already at the correct polarity.
Referring again to FIG. 3, a sheet of support material 138 is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by conventional sheet feeding apparatus, not shown. Preferably, the sheet feeding apparatus includes a feed roll contacting the upper most sheet of a stack of copy sheets. Feed rolls rotate to advance the uppermost sheet from the stack into a chute, which directs the advancing sheet of support material into contact with the surface of belt 110 in a timed sequence, so that the developed toner powder image contacts the advancing sheet of support material at transfer station D.
Transfer station D includes a corona generating device 140 which sprays ions of a suitable polarity onto the backside of sheet 138. This attracts the charged toner powder images from the belt 110 to sheet 138. After transfer, the sheet continues to move in the direction of arrow 142 onto a conveyor (not shown) which advances the sheet to fusing station E.
Fusing station E includes a fuser assembly, indicated generally by the reference numeral 144, which permanently affixes the transferred powder image to sheet 138. Preferably, fuser assembly 144 comprises a heated fuser roller 146 and a backup roller 148. Sheet 138 passes between fuser roller 146 and backup roller 148, with the toner powder image contacting fuser roller 146. In this manner, the toner powder image is permanently affixed to sheet 138. After fusing, a chute, not shown, guides the advancing sheet 138 to a catch tray (also not shown), for subsequent removal from the printing machine by the operator.
After the sheet of support material is separated from the photoconductive surface of belt 110, the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station F. A magnetic brush cleaner housing 150 is disposed at the cleaner station F. The cleaner apparatus comprises a conventional magnetic brush roll structure for causing carrier particles in the cleaner housing to form a brush-like orientation relative to the roll structure and the charge retentive surface. It also includes a pair of detoning rolls for removing the residual toner from the brush.
Subsequent to cleaning, a discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining, prior to the charging thereof, for the successive imaging cycle. An illustrative example of a quad-level xerographic unit is found in U.S. Pat. No. 4,731,634, commonly assigned with this application and herein incorporated by reference.
A quad-level xerographic unit, unlike the bi-level and tri-level, does not produce color images that match the toner colors. Two of the toner colors are produced while the third color produced is a combination of one of those first two toner colors and a third toner color.
A raster output scanner (ROS) optical system for creating tri-level exposures at a recording medium typically prints black, white, and a single color. A raster output scanner optical system for creating quad-level exposures at a recording medium typically prints black, white, and two colors. However, a full color process would print the six primary colors of cyan, yellow, magenta, blue, green, and red, in addition to black and white.
It is an object of this invention to provide a color printing system using a quad-level xerographic unit.
It is another object of this invention to provide a color printing system using a quad-level xerographic unit and a tri-level xerographic unit.
It is still another object of this invention to provide a full color printing system.
It is still another object of this invention to provide a single pass color printing system which will increase the pages per minute printing and will reduce the number and cost of optical and xerographic components.